1. Field of the Invention
The present invention relates to an electron source forming substrate used in forming an electron source and to the electron source and an image forming apparatus using the substrate.
2. Related Background Art
Heretofore in the past, there have been known electron-emitting devices, which are broadly classified into two types using thermionic electron-emitting devices and cold cathode electron-emitting devices. For the cold cathode electron-emitting devices, there are available field emission type (hereinafter referred to as “FE type”), metal/insulating layer/metal type (hereinafter referred to as “MIM type”), surface conduction type electron-emitting device and the like.
As an example of the FE type, there are known those devices as disclosed in W. P. Dyke & W. W. Dolan, “Field emission”, Advance in Electron Physics, 8.89 (1956) or C. A. Spindt. “Physical properties of Thin-Film Field Emission Cathodes with Molybdenium Cones”, J. Appl. Phys., 47,5248 (1976) and the like.
As an example of the surface conduction type electron-emitting device type, there are known those as disclosed in M. I. Elinson, Recio Eng. Electron Phys., 10,1290 (1965) and the like.
The surface conduction type electron-emitting device utilizes a phenomenon where electron emission occurs by letting current flow in parallel with a film surface on a thin film of a small area formed on a substrate. For the surface conduction type electron-emitting device, there are reported those which use SnO2 thin film by the above described Elinson and the like, Au thin film “G. Dittmer: “Thin solid Films”, 9, 317 (1972)”, In2O3/SnO2 thin film “M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.” 519 (1975)”, carbon thin film “Hisashi Araki et. al: SHINNKUU Vol. 26, No. 1, 22 pages (1983)” and the like.
To utilize the above described electron-emitting device by holding an electron source arranged and constructed on a substrate inside an envelope which is kept vacuum inside, it is necessary to connect the electron source to the envelope and other members. This connection is usually performed by heating and fusion by using frit glass. The heating temperature at this time is typically approximately 400° C. to 500° C. and the time thereof is typically approximately ten minutes to one hour, which differs depending on the size of the envelope.
For the material of the envelope, in view of simplicity and reliability of the connection by frit glass and relatively inexpensive cost, soda lime glass is preferably used. Also, because a high strain point glass where a strain point is raised by replacing a part of Na by K is easy for frit connection, it can be preferably used as well. Also, with regard to the material of the substrate of the above described electron source, in view of reliability of the connection to the envelope, similarly the soda lime glass or the above described high strain point glass is preferably used.
The above described soda lime glass contains alkali element metal as its component and particularly contains the large volume of Na as Na2O. Na element is easy to diffuse by heat and, therefore, when exposed to high temperatures during a processing, Na is sometimes diffused into each type of members formed on the soda lime glass, particularly into the member constituting the electron-emitting device, thereby deteriorating its characteristics.
It was reveled that the above described influence by Na sometimes occurs, but to a lessened degree, by that much if Na content is small when the above described high strain point glass is used as the substrate of the electron source.
As means for reducing the above described influence of Na, for example, there are disclosed in Japanese Patent Application Laid-Open No. 10-241550, EP-A-850892 an electron source forming substrate where the concentration of Na of the surface area of the side where the electron-emitting device of the substrate containing Na is at least arranged is smaller than that of other area and also an electron source forming substrate having a phosphorus containing layer.
On the other hand, however, since the substrate where the electron source is formed usually comprises the insulating material, when it is driven in a state applied with a high voltage which is used for discharging electron, a charge-up phenomenon occurs in the part where the substrate is exposed, and when there is no measure taken for this charge-up, a steady and long duration drive becomes difficult. Besides the locus of electron discharged from the electron source is disturbed and there are some cases where the elapsed time changes occur in the electron emitting characteristics.
As means for reducing the influence attributable to the charge-up described as above, for example, there are disclosed in U.S. Pat. No. 4,954,744 or Japanese Patent Application Laid-Open No. 8-180801 that the substrate surface or the electron-emitting device surface is covered by an antistatic film having a sheet resistance of 108–1010 Ω/▭.